Did a quick Google search as a refresher, and the red stems do not indicate deficiencies. They can come along with other deficiencies, but it's more likely it's something else very minor.
Are the bigger fan leaves only yellowing? Top or bottom of the plant?
The picture you showed here, if the leaves are showing yellowing and dark rusty spots, that's magnesium deficiency. If the leaves stay green and show lighter rust spots, that's calcium deficiency.
Stop fretting ya daft c*nt, don't flush
In all seriousness, I don't see the need to flush at the moment. If you think maybe they have some built up salts in between feedings, you could always start doing a feed-feed-water regime. Every 3rd time they need a drink, just use ph water with a little run off. That should clear up some built up salts and let the plants use up some of the leftover nutes in the medium.. the plants will tell you if they like it.
Genetics likely, those bbm look just fine to me right now.